1. Field of the Invention
The present invention relates to thermal transfer ink to be used for thermal transfer printing by a thermal head. The present invention also relates to a thermal transfer printing medium which employs said thermal transfer ink.
2. Description of the Prior Art
The thermal transfer printing system is now in general use for computer printers, word processors, facsimiles, and copying machines. It employs a thermal transfer printing medium 10 (shown in FIG. 1) which consists of a substrate sheet 1, and a release layer 2 and an ink layer 3 laminated consecutively on one side of the substrate, and a protective layer 4 laminated on the other side of the substrate. When the thermal transfer printing medium 10 is heated by a heat-generating element such as thermal head in contact with the protective layer 4, the ink layer 3 is fused and the fused ink is transferred to printing paper on which images are to be made. The release layer 2 ensures the smooth transfer of ink to printing paper with a small amount of energy. The protective layer 4 permits the thermal transfer printing medium to run smoothly without sticking to the thermal head.
The thermal transfer printing medium 10 has an ink layer 3 which is conventionally a fusible thermal transfer ink composed of a colorant and a wax-based binder. The conventional thermal transfer ink gives transfer images which are poor in heat resistance and rub resistance. To eliminate this disadvantage, attempts are being made to use a thermoplastic resin as the principal component of the binder. Such attempts, however, have posed another problem associated with the "cutting-off" of the ink layer at the time of transfer. In other words, the ink layer containing a binder of thermoplastic resin does not cut sharp at the boundary between the heated part and the unheated part. This results in blurred transfer images.